Transparent substrate with antireflection coating

ABSTRACT

The invention relates to a glass substrate having on at least one of its faces an antireflection coating formed by a stack of thin dielectric material layers having alternately high and low refractive indices. To prevent the modification of the optical properties of the coating in the case where the substrate is subject to a heat treatment such as tempering, bending or annealing, the layer or layers of the stack which are liable to deteriorate on contact with alkali ions such as sodium ions are separated form the substrate by at least one layer forming part of the antireflection coating and forming a “shield” with respect to the diffusion of alkali.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to transparent and in particular glass substrates,which are provided with an antireflection coating, as well as to theirproduction method. It also relates to the use thereof, particularly asglazings.

2. Discussion of the Background

An antireflection coating is usually formed by a stack of thin interfacelayers, generally an alternation of low and high refractive indexlayers. When deposited on a transparent substrate, such a coating hasthe function of reducing its light reflection, i.e. of increasing itslight transmission. Thus, a substrate coated in this way is subject toan increase in its transmitted light to reflected light ratio, whichimproves the visibility of objects positioned behind it.

An antireflection coating can then be used in numerous applications,e.g. for protecting a panel illuminated by a light placed behind theobserver, or for forming or constituting part of a shop display window,so as to make it easier to see what is in the window, even when theinternal illumination is weak compared with the external illumination.

The performance characteristics of an antireflection coating can bemeasured or evaluated on the basis of different criteria. Clearly thefirst criteria are of an optical nature. It can be considered that a“good” antireflection coating must be able to lower the light reflectionof a standard clear glass substrate to a given value, e.g. 2%, or even1% and less. It can also be important that the coating ensures that thesubstrate retains a satisfactory, e.g. neutral calorimetry, very closeto that of the bare substrate. Other secondary criteria can be takeninto account as a function of the envisaged application, particularlythe chemical and/or mechanical durability of the coating, the cost ofthe materials used or the methods to be used for producing the same.

Patent application WO-92/04185 discloses an antireflection coatingdeposited on a transparent substrate and constituted by an alternationof layers having a high niobium oxide index and a low silicon oxideindex. Its optical performance characteristics are interesting. It isadvantageous to use niobium oxide from the industrial standpoint,because it is a material which can be deposited faster than other highindex oxides of the titanium oxide type using known vacuum methods, suchas reactive cathodic sputtering. However, it is found that such a stackis sensitive to any heat treatment and at high temperature its opticalproperties are unfavorably modified, particularly with respect to itscolorimetry in reflection. This is disadvantageous if it is wished togive the particular substrate already provided with its coatingmechanical or esthetic properties which can only be obtained by heattreatments at temperatures which may approach the softening temperatureor point of the glass. Such treatment can, e.g., consist of bending orgiving the substrate a certain curvature, an annealing for hardening it,or a tempering to prevent injury in the case of shattering.

One object of the invention is to obviate this disadvantage bydeveloping a new type of antireflection, multilayer coating, which hasgood optical performance characteristics and which retains the latter,no matter whether or not the substrate then undergoes a heat treatment.

SUMMARY OF THE INVENTION

The invention relates to a glass substrate having on at least one of itsface an antireflection coating incorporating a stack of thin layers ofdielectric materials with alternatively high and low reflective indices.The invention prevents modification to the optical properties of thecoating in the case where the substrate is subject to a heat treatmentof the tempering, bending or annealing type by ensuring that the layeror layers of the stack which may be subject to deterioration in contactwith alkali metal ions, for example of the sodium ion type, emitted bydiffusion of the substrate are separated from said substrate by at leastone layer forming part of the antireflection coating and which forms a“shield” to the diffusion of the alkali ions.

Thus, it has surprisingly been found that the unfavorable modificationof the optical appearance of antireflection coatings under the effect ofheat was due to the diffusion of alkali ions from the glass, the ionsbeing inserted in at least some of the layers of the coating therebystructurally modifying these layers leading to a deterioration thereof.The solution according to the invention involves not removing from theantireflection coating of any material sensitive to the alkali ions, butinstead isolating the same from the surface of the glass by means of ashielding layer blocking the alkali diffusion process. This layer isalso chosen so as to fulfill, in parallel, an adequate optical functionwithin the antireflection coating. Thus, it is not an additional layerwhich makes the structure of a conventional antireflection coating morecomplicated, which is very advantageous from the industrial standpoint.

Thus, these shielding layers make it possible to produce antireflectioncoatings able to withstand heat treatments without any significantoptical modification, while incorporating materials sensitive to alkali,but offering many other advantages.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows one embodiment of the antireflection stack of the presentinvention in section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Thus, the antireflection coatings according to the invention preferablyincorporate niobium oxide layers as the high refractive index layers(refractive index approximately 2.30), but placed in the coating so asnot to be in contact with the alkali ions of the glass. Niobium oxide(Nb₂O₅) is an interesting material which, as stated hereinbefore, can berelatively easily deposited by reactive cathodic sputtering, which has asufficiently high refractive index and, in particular, a satisfactorymechanical durability.

The antireflection coatings according to the invention may also compriselayers of alkali-sensitive materials other than niobium oxide. It is infact possible to use tungsten oxide, which has a high refractive index(index approximately 2.17), whose optical appearance can be modified bythe insertion of sodium ions. This also applies with respect to ceriumoxide (CeO₂). Bismuth oxide (Bi₂O₃) can also be used and has a highindex (approximately 2.30 to 2.52), as well as oxides having multiplevalencies.

Preferably, the antireflection coatings according to the invention aresuch that, on the one hand, the low index dielectric material layershave a refractive index between 1.35 and 1.70, preferably between 1.38and 1.68, and the high index dielectric material layers with arefractive index of at least 1.80 and preferably between 1.80 and 2.60,more preferably between 2.0 and 2.43, e.g. between 2.10 and 2.35. Theantireflection effect is only fully obtained if there is a significantrefractive index difference between the high and low index layersarranged in alternating manner.

There are several embodiments of the shielding layer according to theinvention. In general terms, the closer this layer is to the surface ofthe glass, the more it will be able to rapidly stop the diffusion ofalkali ions through the stack.

Preferably, the shielding layer is one of the low index layers of thestack and in particular the first low index layer, i.e. that closest tothe glass. Also preferably, the first layer has an optical thicknessbetween 40 and 70 nm, particularly approximately 45 to 60 nm. It can beconstituted by different materials, all of which have a low index andstop the migration of alkalis and which are in particular chosen fromamong silicon oxide (SiO₂), doped aluminum oxide of the Al₂O₃:F type ora mixture of these compounds (the term “doping” here means that thefluorine level in the layer is adequate to lower the refractive index ofthe alumina to values permitting its use as a low index layer).

An antireflection coating usually has as the first layer a high indexlayer. When the shielding layer is a low index layer, it is consequentlyimportant that the high index layer on which it is generally placed ismade from a material able to maintain essentially the samecharacteristics, particularly optical characteristics, following heattreatment. However, although the material of the high index layer mustnot deteriorate in contact with the alkali ions, it may still undergo aslight crystallographic modification, particularly when this has noharmful repercussions on its optical properties. Materials such as tinoxide (SnO₂), which may optionally be doped, zinc oxide (ZnO), tantalumoxide (Ta₂O₅) or zirconium oxide (ZrO₂) are suitable.

Another embodiment consists of choosing as the shielding layer a highrefractive index layer, particularly the first layer in contact with theglass. This also protects all the other layers of the stack against theaction of alkali ions. This shielding layer preferably has an opticalthickness between 25 and 50 nm and can be chosen from silicon nitride(Si₃N₄) or aluminum nitride (AlN), both materials having an index closeto 2.0 and which block alkali ions and are inert with respect thereto.

An antireflection coating according to the invention may only comprisetwo successive sequences of high and low index layers. Thus, four layersmay be sufficient to obtain a remarkable antireflection action. In thiscase, the first sequence must comprise the shielding layer (either a lowor a high index layer) and the second sequence comprises tungsten,bismuth or niobium oxide in particular with an optical thickness between245 and 290 nm, as well as a final, low index layer of the SiO₂ type ora mixture of aluminum-silicon oxide, particularly with an opticalthickness between 120 and 150 nm.

An example of this configuration is the following stack:

glass/SnO₂/SiO₂/Nb₂O₅/SiO₂ or

glass/SnO₂/SiO₂/Bi₂O₃/SiO₂ or

Glass/SnO₂/SiO₂/WO₃/SiO₂,

the SiO₂ shielding layer protecting the Nb₂O₅ layer which covers it, theSnO₂ layer remaining inert to the alkali ions and not deterioratingunder the effect of heat. It is obvious that this type of stack can alsohave six layers, with a third high/low index oxide sequence.

In another configuration, the second sequence of the antireflectionstack according to the invention can comprise an overall high indexlayer. The term “overall” means that there is a superimposing of highindex layers, namely two or three layers, where at least one layer is ofniobium, tungsten or bismuth oxide. The following stack is an example ofthis configuration:

glass/SnO₂/SiO₂/Bi₂O₃/SnO₂/Bi₂O₃/SiO₂

or glass/SnO₂/SiO₂/Nb₂O₅/SnO₂/Nb₂O₅/SiO₂.

According to a third embodiment, the shielding layer according to theinvention is completely substituted for the first sequence of high andlow index layers and has an intermediate refractive index between 1.7and 1.8. It preferably has an optical thickness between 80 and 120 nm.Such an intermediate layer has an optical effect very similar to that ofa high/low index layer sequence and has the advantage of reducing thetotal number of layers in the stack. It is advantageously based on amixture of silicon and tin/silicon and zinc/silicon and titanium oxide,or can be based on silicon oxynitride. The relative proportion betweenthe different constituents of these materials makes it possible toadjust the refractive index of the layer. Silicon oxynitride(SiO_(x)N_(y)) is known in the art and can be prepared by reactivecathodic sputtering using a silicon or doped silicon target in thepresence of oxygen and nitrogen gas. The relative amounts of oxygen (x)and nitrogen (y) are adjusted by changing the ratio of oxygen gas tonitrogen gas. The specific stoichiometry can be readily selected by onehaving ordinary skill in this art by varying the oxygen and nitrogen gasratio.

A stack configuration example using such a shielding layer is asfollows:

glass/SiO_(x)N_(y)/Nb₂O₅/ SiO₂.

Here again the SiO_(x)N_(y) shielding layer protects in an effectivemanner the Nb₂O₅ layer covering it.

No matter which embodiment is chosen, the invention permits theproduction of glass substrates carrying an antireflection stack having alight reflection R_(L) of at the most 2%, preferably at the most 1%, thereflection being maintained at 0.5%, or even to within 0.3%,particularly to within 0.2% and even ±0.1% if the glass substrate thenundergoes a heat treatment such as bending, tempering, or annealing.

In the same way, the colorimetry in reflection remains virtuallyunchanged (particularly in the blue or blue-green shades) with,according to the calorimetric system (L*, a*, b*), variations of a* andb* in reflection of at the most 2, particularly at the most 1.5 inabsolute values. In overall manner, the best treatments bring about nodeterioration of the optical appearance in reflection of this type ofantireflection stack, when using as a reference the sensitivity of thehuman eye.

This leads to a series of advantages, namely a single antireflectioncoating configuration is sufficient for producing glazings which may ormay not be bent and may or may not be tempered.

It becomes unnecessary, on the one hand, to have a type of coating withno alkali-sensitive layers for substrates which undergo heat treatment,and on the other hand, a coating type which can have a type of layer,e.g. of Nb₂O₅, for substrates which are not to undergo heat treatment.This facilitates the management of stocks and makes it possible to veryrapidly adapt production to treated or untreated glazings, as required,without having to worry about the antireflection coating type.

Another advantage is that it is possible to assemble in random manner ona building facade, e.g. in a display window, glazings havingantireflection coatings, certain of which are and certain of which arenot heat treated. The eye is unable to detect the disparity in theoverall optical appearance of the glazing assembly.

It also becomes possible to sell non-heat treated coated glazings,leaving it to the purchaser to heat treat them, whilst being able toguarantee a consistency in their optical properties.

Preferably, each of the glass substrate faces is coated with anantireflection stack according to the invention, in order to obtain themaximum antireflection effect.

According to the invention, at least one of the low index layers of theantireflection stack can be based on a silicon-aluminum oxide mixture(optionally fluorinated), particularly the last layer of the stack. Sucha mixed oxide layer has in particular a chemical durability which isbetter than a pure SiO₂ layer. The optimum aluminum level in the layeris chosen so as to obtain this improved durability, but withoutexcessively increasing the refractive index of the layer compared withpure silica and so as not to deteriorate the optical properties of theantireflection system, alumina having an index of approximately 1.60 to1.65 higher than that of SiO₂, which is approximately 1.45.

The invention also relates to glazings incorporating coated substrates,no matter whether they are monolithic, laminated or multiple withinterposed gas layers.

These glazings can be used both as internal and external buildingglazings, and as protective glass for objects such as panels, displaywindows, glass furniture such as a counter, a refrigerated display case,etc. also as car glazings such as laminated windshields, mirrors,antiglare screens for computers and decorative glass.

The glazing incorporating the antireflection coating substrate accordingto the invention may have interesting additional properties. Thus, itcan be a glazing having a security function, such as the laminatedglazings marketed by SAINT-GOBAIN VITRAGE under the name STADIP, ortempered glazings such as those marked by SAINT-GOBAIN VITRAGE under thename SAKURIT. They can also be burglarproof glazings, such as thosemarketed by SAINT-GOBAIN VITRAGE under the name CONTRARISC, orsoundproofing glazings such as those marketed by SAINT-GOBAIN VITRAGEunder the name CONTRASONOR (double glazings) or PHONIN (laminatedglazings) or also as fire protection glazings (fire-screen orfire-proof).

The glazing can also be chosen in such a way that on the substrate,already provided with the antireflection stack, or with otherglazing-forming substrates on one of its faces, is deposited a layer (ora stack of layers) having a specific function, e.g., sun-shielding orheat-absorbing, such as titanium nitride layers, or layers such as thosemarketed under the name COOL-LITE or ANTELIO or COOL-LITE K bySAINT-GOBAIN VITRAGE, or also having an anti-ultraviolet, antistatic(such as slightly conductive, doped metallic oxide layer) andlow-emissive, such as silver-based layers of the PLANITHERM type ordoped tin oxide layers of the EKO type marketed by SAINT-GOBAIN VITRAGE.In the case of an antistatic function layer, it is preferable for thelatter to be placed on the substrate face provided with theantireflection stack. The layer can also be of the heating type (metallayer with adequate current leads), which can be of interest forrefrigerated display cases, for preventing the deposition of mist ontheir surface. It can also be a layer having anti-soiling propertiessuch as a very fine TiO₂ layer, or a hydrophobic organic layer with awater-repellent function or hydrophilic layer with an anti-mistfunction. An example of a hydrophobic layer is the fluorinatedorganosilane-based layer described in U.S. Pat. Nos. 5,366,892 and5,389,427 incorporated herein by reference.

The layer can be a silver coating having a mirror function and allconfigurations are possible. Thus, in the case of a monolithic glazingwith a mirror function, it is of interest to deposit the antireflectioncoating on face 1 (i.e., on the side where the spectator is positioned)and the silver coating on face 2 (i.e., on the side where the mirror isattached to a wall), the antireflection stack according to the inventionthus preventing the splitting of the reflected image.

In the case of a double glazing (where according to convention the facesof glass substrates are numbered starting with the outermost face), itis thus possible to place the antireflection stack on face 1 and theother functional layers on face 2 for anti-ultraviolet or sun-shieldingand 3 for low-emissive layers. In a double glazing, it is thus possibleto have at least one antireflection stack on one of the faces of thesubstrates and at least one layer or a stack of layers providing asupplementary functionality. The double glazing can also have severalantireflection coatings, particularly at least on faces 2 or 3. For amonolithic glazing 1 it is possible to deposit an antistatic functionlayer, associated with a second antireflection stack.

In the same way, the glass chosen for the substrate covered with thestack according to the invention or for other substrates associatedtherewith for forming a glazing can in particular be, for example,extra-clear of the PLANILUX type or tinted of the PARSOL type, boththese products being marketed by SAINT-GOBAIN VITRAGE. The glass canitself have a filtering function with respect to ultra-violet radiation.The substrate or substrates can undergo heat treatments, which theantireflection stack according to the invention is able to withstand,such as tempering, bending or even folding, i.e., a bending action witha very small radius of curvature (application to shop counters).

The substrate may also undergo a surface treatment, particularly agrinding (frosting), the antireflection stack being depositable on theground face or on the opposite face.

The substrate, or one of those with which it is associated, can also beof the printed, decorative glass type, such as ALBARINO, marketed bySAINT-GOBAIN VITRAGE, or can be screen process printed.

A particularly interesting glazing incorporating the substrate withantireflection coating according to the invention is a glazing having alaminated structure with two glass substrates combined by apolyvinylbutyral (PVB)-type assembly polymer sheet. At least one andpreferably both substrates are provided with antireflection coatingsaccording to the invention, preferably on the outer face and inparticular with the sequence: antireflectioncoating/glass/PVB/glass/antireflection coating.

This configuration, particularly with two bent and/or temperedsubstrates, makes it possible to obtain a car glazing and in particulara windshield of a very advantageous nature. Thus, the standards requirecare to have windsheilds with a high light transmission of at least 75%in normal incidence. Due to the incorporation of antireflection coatingsin a laminated structure of a conventional windshield, the lighttransmission of the glazing is improved, so that its energy transmissioncan be slightly reduced, while still remaining within the lighttransmission standards. Thus, the sun-shielding effect of the windshieldcan be improved, e.g., by absorption of the glass substrates. The lightreflection value of a standard, laminated windshield can be brought from8% to less than 1%, while decreasing its energy transmission from 1 to10%, e.g., by passing it from 85 to 81%.

The invention also relates to a process for the production of glasssubstrates having an antireflection coating. The process involvesdepositing the layers in succession using a vacuum process, particularlymagnetic field-assisted cathodic sputtering. It is thus possible todeposit oxide layers by reactive sputtering of the metal in question inthe presence of oxygen and the nitride layers in the presence ofnitrogen.

Another choice involves depositing all or part of the layers of thestack, particularly the first layer or layers, by pyrolysis ofappropriate precursors. It can, in fact, be a solid phase pyrolysisusing precursors in powder form (e.g., tin dibutyl difluoride forforming tin oxide), e.g., in liquid form by dissolving the precursor orprecursors in a solvent, or gaseous form. In the latter case, theprecursor is brought into gaseous form. It can be tetraorthosilicate(TEOS) or SiH₄ for forming silicon oxide. The pyrolysis may take placedirectly and continuously on the hot float glass ribbon, the followinglayers then being subsequently deposited on the already cut glass usinga cathodic sputtering method.

The details and advantageous characteristics of the invention willbecome apparent from the non-limitative examples given hereinafterrelative to FIG. 1. The very diagrammatic FIG. 1 shows in section asubstrate surmounted by an antireflection stack according to theinvention (the proportions between the thickness of the substrate andthose of the layers are not shown to facilitate understanding). Thus,each of the faces of the substrate is provided with an identical stack,but only one is shown for reasons of clarity. The use of a coating oneach of the substrate faces is provided in all the following examples.

It is pointed out that in these examples, the successive deposits ofthin layers take place by magnetic field-assisted reactive cathodicsputtering, but it would also be possible to use any other vacuumprocess or a pyrolysis process permitting good control of thethicknesses of the layers obtained.

The substrates on which are deposited the antireflection coatings areclear soda-lime-silica glass substrates of the PLANILUX type, with athickness of 3 to 6 mm and in particular 4 mm.

FIG. 1 shows the glass substrate 1 coated, according to a firstembodiment, on its two faces with a four-layer stack 6 consisting of analternation of high index thin layers 2, 4 and low index thin layers 3,5. Another embodiment consists of replacing the two layers 2, 3 by anintermediate index layer 7.

Other features of the invention will become apparent in the course ofthe following descriptions of exemplary embodiments which are givenbelow for illustration of the invention but are not intended to belimiting thereof.

EXAMPLES COMPARATIVE EXAMPLE

This example uses a four-layer coating consisting of the followingsequence:

glass/Nb₂O₅/SiO₂/Nb₂O₅/SiO₂.

The two Nb₂O₅ layers with a refractive index of approximately 2.3 wereobtained by reactive sputtering in the presence of oxygen from niobiumtargets, the two SiO₂ layers of refractive index approximately 1.47 areobtained by reactive sputtering in the presence of oxygen from silicontargets doped with boron or aluminum.

The following Table 1 gives the geometrical thickness in nanometers ofeach of the layers of the stack, numbered in accordance with FIG. 1.

TABLE 1 COMPARATIVE EXAMPLE Nb₂O₅ (2) 12 SiO₂ (3) 38 Nb₂O₅ (4) 120  SiO₂(5) 87

Example 1 According to the Invention

This example uses a four-layer, antireflection coating in accordancewith the following sequence:

glass/SnO₂/SiO₂/Nb₂O₅/siO₂.

The last three layers were obtained, as described above and with thesame refractive index. The first layer was obtained by reactive cathodicsputtering in the presence of oxygen from a tin target and its index isapproximately 2.

The following Table 2 gives, for each of the layers of the stack,numbered in accordance with FIG. 1, the preferred geometrical thicknessrange in nm, as well as its precise thickness selected from within therange.

TABLE 2 EXAMPLE 1 ACCORDING TO THE INVENTION Preferred range ThicknessSnO₂ (2) 10-30 19 (15-25) SiO₂ (3) 25-40 33 (30-38) Nb₂O₅ (4) 100-150115  (110-130) SiO₂ (5)  70-100 88 (80-90)

The substrates coated according to the comparative example and example 1then underwent an annealing treatment consisting of heating for one hourat 550° C.

The following Tables 3 and 4 give, for each of the two substrates,before and after the heat treatment, the following photometric data:

light reflection value R_(L) in %, according to illuminant D₆₅, undernormal incidence,

values of a*_((R)), b*_((R)) and L*_((R)) in reflection, without units,according to the colorimetry system (L, a*, b*).

TABLE 3 COMPARATIVE EXAMPLE Before heat treatment After heat treatmentR_(L) 0.7 0.5 a*_((R)) −2.8 3.5 b*_((R)) −0.0 −2.0 L*_((R)) 6.5 5.0

TABLE 4 EXAMPLE 1 ACCORDING TO THE INVENTION Before heat treatment Afterheat treatment R_(L) 0.55 0.66 a*_((R)) −6.55 −7.94 b*_((R)) −0.47 +0.89L*_((R)) 4.98 5.98

In addition, two substrates were taken, each provided on one of theirfaces with the stack defined in Table 2, with assembly by a standard PVBsheet in order to produce a windshield in accordance with the followingsequence:

coating (6)/glass/PVB/glass/coating(6).

Compared with the same sequence, but without the two antireflectioncoatings (6), the R_(L) is 0.45 instead of 8.15 (invention) and theenergy transmission I_(E) is 81.5% instead of 85% (invention).

Example 2 According to the Invention

This example uses a three-layer, antireflection coating with thefollowing sequence:

glass/SiO_(x)N_(y)/Nb₂O₅/SiO₂.

The last two layers are formed like the Nb₂O₅ and SiO₂ layers of thepreceding examples. The first layer is obtained by reactive cathodicsputter in the presence of an O₂/N₂ atmosphere from a boron oraluminum-doped silicon target.

The SiO_(x)N_(y) layer has a refractive index of approximately 1.75.Table 5 gives for each of the three layers, their preferred geometricalthickness ranges, as well as their precise thicknesses (nm).

TABLE 5 Preferred range Thickness SiO_(x)N_(y) (7) 45-75 61 (55-65)Nb₂O₅ (3)  90-130 104  (100-110) SiO₂ (4)  70-100 86 (80-90)

The substrate is able to withstand the same type of heat treatment asthat undergone in the preceding examples, without any significantmodification of its appearance in reflection.

Example 3 According to the Invention

Like Example 1, this example uses a four-layer, antireflection coatingwith the following stack:

glass/SnO₂/SiO₂/Bi₂O₃/SiO₂.

The bismuth oxide is deposited by reactive cathodic sputtering from abismuth target.

Table 6 gives for each of the layers their preferred geometricalthickness ranges and their precise geometrical thicknesses innanometers.

TABLE 6 EXAMPLE 3 Preferred range Thickness SnO₂ (2) 10-30 21 (15-25)SiO₂ (2) 20-35 28 (25-32) Bi₂O₃ (4)  80-130 108   (95-115) SiO₂ (5) 70-110 86 (80-96)

The light reflection R_(L) of the thus coated substrate is 0.50%. Thevalues of a*_((R)) and b*_((R)) in reflection are, respectively,approximately −3 and approximately −1.

Example 4 According to the Invention

This example uses a six-layer, antireflection stack. The second highindex layer starting from the substrate is, in fact, formed from threeoxide layers with an index equal to or higher than 2. The stack is asfollows (geometrical thicknesses in nanometers given beneath each of thelayers):

The light reflection R_(L) of the coated substrate is 0.45%. The valuesof a* and b* in reflection are, respectively, approximately −3 and −1.

Example 5 According to the Invention

This example uses a five-layer, antireflection stack, the second highindex layer starting from the substrate being constituted by twosuperimposed oxide layers with an index above 2. The stack is as follows(the same conventions regarding thicknesses as in example 4):

The light reflection of the substrate is then 0.60% with values of a*and b* in reflection of approximately −3 and −1.

Example 6 According to the Invention

This example uses an antireflection stack with a structure similar tothat of Example 5, while reversing the sequence of the two superimposed,high index layers, so that the stack is as follows:

The light reflection of the substrate is about 0.50% with values of a*and b* in reflection of approximately −3 and −1.

It is pointed out that in examples 4 to 6, the thicknesses of each ofthe constituent layers of the antireflection stacks have been selectedso as to obtain in reflection a colorimetry corresponding to negativevalues of a* and b* and, in absolute values, not too high, which meanscolors in reflection in the blue-green which are agreeable and not veryintense. It is obvious that there is no departure from the scope of theinvention when choosing similar stack structures, but with slightlydifferent layer thicknesses, e.g. 10 to 20% thicker or thinner. It isthus possible, by adapting the thicknesses, to adapt the colorimetry inreflection as a function of need.

It must also be stressed that the stacks of examples 3 to 6 are able towithstand, without any significant optical modification, the heattreatment undergone by the stack of example 1.

It should also be noted that it is advantageously possible to replacethe last SiO₂ layers of the stacks of the examples according to theinvention by layers of mixed aluminum-silicon oxide, so as to make thelayer harder and in particular more chemically resistant (moistureresistance), which is of interest if the antireflection stack has to beplaced on the outer face of a glazing. However, the aluminum level mustbe controlled, so as not to excessively increase the refractive index ofthe layer. A level of, for example, 2 to 12 wt. % aluminum, based on theSiO₂, is satisfactory.

The following conclusions can be drawn from all these results. It ispossible to see from Table 4:

the antireflection coatings according to the invention give the glasssubstrates very low light reflection values below 1% (to be comparedwith the light reflection of approximately 8% which these substrateswould have without coating),

their color in reflection is also very neutral, particularly very low inthe blue-green with respect to example 1, which is a presently sought,aesthetic shade, particularly for glazings for buildings,

their optical characteristics undergo little or no modification, whenthe substrates have undergone a high temperature treatment, this moreparticularly applying to their appearance in reflection.

Thus, the variation in the value of R_(L), designated ΔR_(L), is of aminimum nature, generally below 0.3% and approximately 0.1%. What iseven more important, is that their favorable colorimetry in reflectionis maintained. The variation of the factor a*, designated Δa*, is inabsolute values generally below 2.0, particularly 1.36. The variation ofthe factor b*, designated Δb*, is of the same order of magnitude. Oncalculating the value of ΔE on the basis of the data in Table 4, thevalue being defined by {square root over ({square root}ΔL²+a²+b²)}, i.e.the square root of the sum of the squares of the variations of a* and b*and L*, a value generally below 3 and in particular 2.2 is obtained,this value reaching the sensitivity limits of the human eye, which isunable or just able to distinguish between a substrate having heattreated, antireflection stacks and the same, untreated substrate and inboth cases remaining in a blue-green shading.

This is not the case with comparative example 1. On referring to Table3, it can be seen that the heat treatment significantly modifies theappearance in reflection of the substrate. In the example according tothe invention, the blue-green shade is maintained, whereas in thecomparative example the color in reflection swings from green tomauve/violet, the latter shade not being aesthetically appreciated. Thiscolor change can be seen by an observer. On calculating for thiscomparative example the value of ΔE, as defined hereinbefore, a value ofapproximately 7.7 is obtained, which falls within the sensitivity rangeof the human eye.

The reasons for this appearance change are that the first, niobium oxidelayer, i.e. the layer nearest to the glass, suffers the effect of thediffusion of the sodium ions (Na⁺) at high temperature, a verysignificant structural modification, being transformed into a mixedcompound with sodium and niobium and no longer having the initial oxideproperties. The same applies for bismuth or tungsten oxide, tungstenoxide also being known to color highly as a result of the insertion ofsodium ions.

Thus, the invention makes it possible to establish a compromise, byretaining in its antireflection stacks oxides of the type Nb₂O₃, WO₃,CeO₂ or Bi₂O₃, which are sensitive to alkali ions, but isolating themfrom the glass by appropriate shielding layers, with a view to producingglazings which can be safely hardened, bent or tempered following thedeposition of the stacks.

The French priority document FR 95/02102 filed Feb. 23, 1995 isincorporated herein by reference in its entirety.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and is desired to be secured by letters Patent ofthe United States is:
 1. A coated transparent substrate, comprising: atransparent substrate including alkali metal ions; and an antireflectioncoating on at least one surface of said substrate, wherein said coatingis a stack of dielectric material layers having alternating high and lowrefractive indices, at least one of said layers is a shield layer whichprevents diffusion of said alkali metal ions from said substrate throughsaid shield layer, and said shield layer is a high refractive indexdielectric material layer selected from the group consisting of siliconnitride and aluminum nitride, and said coated transparent substrate hasa light reflection R_(L) of at most 2% and a blue or blue-green color inreflection, with a variation of R_(L) of at most 0.3% and variations ofa* and b* in reflection of at most 2.0 after heating.
 2. The coatedtransparent substrate of claim 1, wherein said low refractive indexdielectric material layer has a refractive index between 1.35 and 1.70and said high refractive index dielectric material layer has arefractive index of at least 1.80.
 3. The coated transparent substrateof claim 2, wherein said low refractive index dielectric material layerhas a refractive index between 1.38 and 1.65 and said high refractiveindex dielectric material layer has a refractive index between 1.80 and2.60.
 4. The coated transparent substrate of claim 3, wherein said highrefractive index dielectric material has a refractive index between 2.10and 2.35.
 5. The coated transparent substrate of claim 1, wherein saidhigh refractive index dielectric material layer comprises an oxideselected from the group consisting of niobium oxide, tungsten oxide,bismuth oxide and cerium oxide.
 6. The coated transparent substrate ofclaim 1, wherein said shield layer is a low refractive index dielectricmaterial layer selected from the group consisting of SiO₂, Al₂O₃:F and amixture thereof.
 7. The coated transparent substrate of claim 1, whereinsaid shield layer has an optical thickness of 25 to 60 nm.
 8. The coatedtransparent substrate of claim 1, wherein said antireflection coatingcomprise two successive sequences of high and low refractive indexlayers, a first sequence including said shield layer and a secondsequence including a layer of niobium oxide, bismuth oxide or tungstenoxide, said first sequence and said second sequence also including a lowrefractive index layer of silicon dioxide or a mixture of silicon andaluminum oxides.
 9. The coated transparent substrate of claim 8, whereinsaid shield layer and said layer of niobium oxide, bismuth oxide ortungsten oxide has an optical thickness of 245-290 nm and said lowrefractive index layer has an optical thickness of 120-150 nm.
 10. Thecoated transparent substrate of claim 9, said coating having thestructure SnO₂/SiO₂/Nb₂O₅/SiO₂, SnO₂/SiO₂/Bi₂O₃/SiO₂ orSnO₂/SiO₂/WO₃/SiO₂.
 11. The coated transparent substrate of claim 8,wherein said second sequence comprises a plurality of high refractiveindex layers, wherein at least one of said plurality of layers comprisesniobium oxide, bismuth oxide or tungsten oxide.
 12. The coatedtransparent substrate of claim 11, said coating having the structureSnO₂/SiO₂/Bi₂O₃/SnO₂/Bi₂O₃/SiO₂ or SnO₂/SiO₂/Nb₂O₅/SnO₂/Nb₂O₅/SiO₂. 13.The coated transparent substrate of claim 1, wherein said shield layerhas a refractive index between 1.7-1.8 and an optical thickness of80-120 nm.
 14. The coated transparent substrate of claim 1, wherein atleast one of said low refractive index dielectric material layers is amixture of silicon oxide and aluminum oxide.
 15. The coated transparentsubstrate of claim 14, wherein said at least one low refractive indexlayer containing said mixture is in contact with said substrate.
 16. Thecoated transparent substrate of claim 1, having said coating on aplurality of surfaces.
 17. The coated transparent substrate of claim 1,wherein R_(L) is at most 1%, the variation of R_(L) is at most 0.1% andvariations of a* and b* in reflection are at most 1.5.
 18. A monolithicglazing, laminated glazing or multiple glazing with interposed gaslayer, comprising the coated transparent substrate of claim
 1. 19. Theglazing of claim 18, further comprising sun-shielding, sun-absorbing,anti-ultraviolet, anti-static, low emissive, heating, anti-soiling,security, burglar proof, sound proofing, fire protection, anti-mist,water-repellant or mirror means.
 20. The glazing of claim 18, whereinsaid substrate is frosted, printed or screen process printed.
 21. Theglazing of claim 18, wherein said substrate is tinted, tempered,reinforced, bent, folded or ultraviolet filtering.
 22. The glazing ofclaim 21, wherein said glazing is a car windshield.
 23. The glazing ofclaim 18, having a laminated structure comprising two glass substratesand a polyvinylbutyral layer interposed therebetween, at least one outerface of said glass substrates contacting said coating.
 24. The glazingof claim 23, having the structure: antireflectioncoating/glass/polyvinylbutyral/glass/antireflection coating.
 25. Thecoated transparent substrate of claim 1, wherein said shield layer is incontact with the substrate.
 26. The coated transparent substrate ofclaim 1, wherein said stack comprises an intermediate refractive indexlayer in contact with said substrate and a sequence of layers of highand low refractive indices on said intermediate refractive index layer.27. The coated transparent substrate of claim 1, wherein said coating isa stack of four layers having alternating high and low refractiveindices or a stack of six layers having alternating high and lowrefractive indices.
 28. The coated transparent substrate of claim 1,wherein one of said high refractive index layers is a superposition oftwo or three different layers.
 29. The coated transparent substrate ofclaim 1, wherein said substrate comprises bent glass or tempered glass.30. A process for producing a coated transparent substrate, the processcomprising depositing a first dielectric material layer on a glasssubstrate by pyrolysis of dielectric material precursors; forming asecond dielectric material layer on said first material layer bycathodic sputtering; and forming the coated transparent substrate ofclaim
 1. 31. The coated transparent substrate of claim 1, wherein saidshield layer prevents diffusion of alkali metal ions from said substrateto a layer comprising metallic oxides having multiple valencies.